1. Field of the Invention
The present invention relates to a buried-heterostructure semiconductor laser device which achieves confinement of a current and optical wave-guiding, and also to a method of manufacturing the semiconductor laser device.
2. Description of the Related Art
Semiconductor lasers are increasingly used in optical disk devices such as digital audio-disk (DAD) devices, video-disk (VD) devices, and also in document filing systems. Also, they are used, in increasing numbers, as light sources in optical-fiber communication systems. At present, mode-controlled semiconductor lasers are in general use.
Buried-heterostructure semiconductor lasers are used not only as Fabry-Perot lasers, but also as distributed feedback lasers which have a layer for guiding optical waves. This is because the buried-heterostructure semiconductor laser can be operated efficiently with a relatively small threshold current since a current is confined to the active layer of this semiconductor laser.
FIG. 5 is a cross-sectional view showing a known GaInAsP buried-heterostructure semiconductor laser. This laser is manufactured in the following way.
First, several semiconductor layers are formed on n-(100) InP single-crystal substrate 70 by means of vapor-phase epitaxial growth, liquid-phase epitaxial growth (LPE), or metal organic chemical vapor deposition (MOCVD). More specifically, n-InP buffer layer 71 is formed on substrate 70; optical wave-guiding layer 72 is formed on buffer layer 71; GaInAsP active layer 73 is formed on layer 72; p-InP cladding layer 74 is formed on active layer 73; and p-GaInAsP ohmic layer 75 is formed on clad layer 74. Then, isotropic etching is performed on layers 71, 72, 73, 74, and 75, thereby removing these layers, except for their center portions. As a result of this, a mesa stripe 76 remains on n-(100) InP single-crystal substrate 70. Next, burying layers, which consist of p-InP layer 77 and n-InP layer 78, are formed on the substrate 70. Further, GaInAsP cap layer 79 is formed on the burying layers. Layers 77, 78, and 79 are formed by the same method as the layers of mesa stripe 76. As a result, a buried-heterostructure semiconductor laser is made which has a flat and smooth upper surface because of cap layer 79.
During the process of forming the burying layers (layers 77 and 78) and cap layer 79, mesa stripe 76 is exposed to intense heat for a specific period of time. The heat inevitably damages stripe 76. The thermal damage to active layer 73 may either result in an increase of a leakage current, or deteriorate the operating characteristics of this semiconductor laser.
A buried-heterostructure semiconductor laser of another type is disclosed in Japanese Patent Disclosure No. 60-251687. FIG. 6 illustrates this semiconductor laser. (In this figure, the same numerals designate the same elements as those shown in FIG. 5). As is shown in FIG. 6, the edge portions 80 of active layer 73 are selectively etched and then buried by virtue of mass-transport phenomenon. The exposed edges 81 of active layer 73 are buried by virtue of mass transport. Hence, edges 81 are free from thermal damages which they would otherwise suffer during the process of forming the burying layers. However, positive junction 82 is formed between the mass-transport sections which sandwich active layer 73. Due to this junction 82, part of the current supplied from electrode 83 to electrode 84 does not flow through active layer 73, but via mass-transport sections. In other words, a leakage current increases, and the current cannot be sufficiently confined to active layer 73.